Reference Cited as of interest: U.S. Patent Documents
The present invention relates generally to the elimination of perfluorocompound (PFC) contaminants released with nitrogen gas from a manufacturing chamber through a vacuum pump. The contaminants are eliminated by their exposure to flames of the microwave plasma torch.
PFCs have been widely used in the semiconductor industry for various reasons. These gases have a long atmospheric lifetime. In addition, they have a high global-warming potential because of their intensive absorption of infrared radiation. If the PFC emission continues as it is, they will stay in the atmosphere for a long time, causing the global warming for hundreds to thousand years. Moreover, these gases might have other unknown environmental effects in the future. However, the PFC use in the semiconductor industry has been significantly increased in recent years. In this context, there are international efforts to reduce the emission of PFCs, Hydrogen-containing Fluorocarbons (HFCs) and SF6. For example, the Kyoto Protocol declared the need for significant reduction of the rate of the global-warming gas emission into the atmosphere. The Environmental Protection Agency (EPA) and the semiconductor manufacturing companies signed the Memorandum of Understanding (MOU) in March 1996 in an effort to reduce PFC emission. Therefore, the PFC emission is tightly regulated nowadays. There are four generally acceptable approaches to the reduction of PFC emission. They are the substitution, process optimization, recovery and recycling, and the abatement. Although some alternative chemicals like C3F8 and NF6 are promising as substitutes. They may also have the global warming potential and may result in generation of PFC by-product. Optimization of cleaning and etching processes has made some progress in recent years. But, this optimization has a fundamental limitation. Recycling and recovery of PFCs from the effluent gas may be desirable. But, the current recovery technologies are not economical.
Plasma remediation of the gas emission is an attractive abatement method. A typical plasma abatement device consists of a reactor chamber, in which plasmas are generated by either radio frequency (RF) power source or microwave radiation. The low-pressure plasma abatement system operated by the RF source is directly connected to the semiconductor process line. The coexistence of the low-pressure abatement system with the vacuum process line introduces several troublesome complications in the semiconductor industry. On the other hand, the microwave plasma torch is operated at the atmospheric pressure and is attached to the exit of the vacuum pump operated by the nitrogen gas. The microwave torch abatement system is completely isolated from the semiconductor process line. Moreover, the present invention is compact enough to be integrated with and attached to the dry pump.
Pollution control with respect to contaminated air as a carrier gas, was proposed in U.S. Pat. No. 5,468,356 issued to Uhm, one of the present inventors, on Nov. 21, 1995. In that invention, contaminated air is exposed to microwave-generated plasma for oxidation by atomic oxygen without bulk heating within a simple cylindrical waveguide cavity under room temperature. Further, such plasma is generated within the cavity by introduction of high-power microwave radiation passing through a weak electric field to achieve air purification despite low electron energy. Pollution control with respect to contaminated air as a hot carrier gas, was proposed in U.S. Pat. No. 5,830,328 issued to Uhm, one of the present inventors, on Nov. 3, 1998. The plasma generated in a hot gas like the discharge gas from a combustion engine or like the discharge gas from an incinerator may oxidize the contaminants, purifying the discharge air.
It is therefore an important object of the present invention to enhance the electric field strength of the microwave radiation, in order to achieve elimination of PFC contaminants in a carrier gas by exposure to a plasma torch generated by concentration of the microwave on a small spot.
An additional object is overcome difficulties heretofore experienced in achieving complete elimination of the PFC contaminants by oxidation with a plasma torch.
The present invention is a device for elimination of PFC contaminants discharged from industrial applications. The PFC gases used in the industrial cleaning of high-tech components cause air contamination, including global warming and upper atmospheric pollution. These gases are among the most important global warming agents because they remain in the atmosphere for a long time. The PFC gases used in the cleaning process are chemically stable and therefore difficult to destroy. High electric field strength and high-energy electrons provided by the plasma torch are needed to eliminate the PFC gases.
The present invention is directed to a magnetrons used in home microwave ovens. These magnetrons are inexpensive, commercially abundant and compact. They are operated at a frequency of 2.45 GHz and their power is in the range of 0.6-1.4 kW. Intensity of the microwaves with a frequency of 2.45 GHz from a magnetron increases at discharge tube. These intense microwaves at the discharge tube induce an intense electric field, initiating electrical breakdown in the emission gas containing the PFC contaminants. The plasma torch generated by the electrical breakdown due to the microwave electric field eliminates the PFC contaminants by oxidation and by molecular breakdown. The atmospheric plasma abatement system is suitable for semiconductor cleaning process lines at a handling rate of at least 10 liters per minute of waste gases without vacuum pump assistance. To enhance efficiency of the PFC contaminant elimination, additional reactive gases are mixed with the waste gas which is treated in the plasma reaction chamber, decomposing the perfluorocompounds. Destruction efficiency of the tetrafluoromethane (CF4) is experimentally measured by two analytical equipment""s.